Lubrication fluids are applied between moving surfaces to reduce friction, thereby improving efficiency and reducing wear. Lubrication fluids also often function to dissipate the heat generated by moving surfaces.
One type of lubrication fluid is a petroleum-based lubrication oil used for internal combustion engines. Lubrication oils contain additives that help the lubrication oil to have a certain viscosity at a given temperature. In general, the viscosity of lubrication oils and fluids is inversely dependent upon temperature. When the temperature of a lubrication fluid is increased, the viscosity generally decreases, and when the temperature is decreased, the viscosity generally increases. For internal combustion engines, for example, it is desirable to have a lower viscosity at low temperatures to facilitate engine starting during cold weather, and a higher viscosity at higher ambient temperatures when lubrication properties typically decline.
Additives for lubrication fluids and oils include rheology modifiers, such as viscosity index (VI) improvers. VI improving components, many of which are derived from ethylene-alpha-olefin copolymers, modify the rheological behavior of a lubricant to increase viscosity and promote a more constant viscosity over the range of temperatures at which the lubricant is used. Higher ethylene content copolymers efficiently promote oil thickening and shear stability. However, higher ethylene content copolymers also tend to aggregate in oil formulations leading to extremely viscous formulations. Aggregation typically happens at ambient or subambient conditions of controlled and quiescent cooling. This deleterious property of otherwise advantageous higher ethylene content viscosity improvers is measured by low temperature solution rheology. Various remedies have been proposed for these higher ethylene content copolymer formulations to overcome or mitigate the propensity towards the formation of high viscosity at low temperature.
It is believed that the performance of VI improvers can be substantially improved, as measured by the thickening efficiency (TE) and the shear stability index (SSI), by appropriate and careful manipulation of the structure of the VI improver. Particularly, it has been discovered that performance improves when the distribution of the monomers and the chain architecture are controlled and segregated into at least two compositionally different and/or crystallinity different polymers. These different polymers may be achieved by the use of a synthesis process that employs metallocene-based catalysts in the polymerization process.
One proposed solution is the use of compositions of amorphous and semi-crystalline ethylene-based copolymers for lubricant oil formulations. The combination of two such ethylene copolymers allows for improved thickening efficiency, shear stability index, low temperature viscosity performance and pour point, and “finishing” as pellets for easy dissolving. Traditionally, such copolymer compositions are made from mixing two polymers made from conventional vanadium based Ziegler-Natta catalyst in an extruder or solvent based process. See, e.g., U.S. Pat. Nos. 7,402,235 and 5,391,617, and European Patent 0 638,611, the disclosures of which are incorporated herein by reference.
Similar polymer compositions made with metallocene catalyzed ethylene-α-olefin copolymers can have a tendency to form gels in lubricating oils when stored at low temperatures. Such gelation of metallocene catalyzed copolymers can be observed visually when lubricating oil solutions or poly-alpha olefin (PAO) solutions containing the polymers are cycled from −15° C. to 10° C. or, alternatively, in a low temperature rheological test, where the yield stress and tan δ are measured. Yield stresses in the range of from 0 mPa to 4000 mPa are observed for metallocene catalyzed polymers in PAO solutions containing 2.4 wt % polymer at 0° C. and −15° C. and generally scale with the severity of the gels as rated by the visual gel test method. Further, values of the tan δ scale inversely with the tendency of the solutions to form gels, higher values indicating higher tendency to form non-gelling lubricating oils.
There is a need for processes for making polymer compositions suitable for use as viscosity index improvers for lubricating oils which advantageously form little or no gel at low temperatures.